A lot of nice guns have been mentioned above. But I'd prefer to have the International Harvester Garand that my dad carried in the USMC. I always wanted to find it while he was here, now I just want to find it for the sake of having it.
For something exotic though, my choice would be either something one of a kind, or serial number one. Too many choices to make a decision since it ain't gonna happen. But it would be nice to have SN#1 in either a Colt 1873 SAA, Colt 1911, Winchester Model 1866, Henry rifle, etc. Like I said there's too many fine firearms that have been made to choose just one.
Wyman